This invention relates to an improved ion permeable membrane, and more particularly to an improved battery electrode separator material and to a method of making it. Improved electrochemical cells of various types are made possible with this separator material.
Ion permeable materials have been used for a long time as electrode separators and half cell dividers for electrochemical cells. Early ion permeable battery separators were made of wood, paper and other cellulosic type materials. However, these types of materials suffer from many disadvantages, including that they tend to degrade with age. Accordingly, ion permeable battery separators made with other materials, including synthetics, have been developed. Separators have been made of asbestos. They have also been made of sintered polytetrafluoroethylene particles, as well as with sintered polyethylene.
In general, separators made with synthetic materials are improved in that they are more resistant to chemical attack. On the other hand, they are less wettable. Moreover, when made with a suitably small pore size, they tend to present greater electrical resistance between cell electrodes than natural materials. If made sufficiently thin to reduce this electrical resistance, they may become mechanically weak, dimensionally unstable, and more subject to electrode dendrite penetration.
We have now found an improved material for use as an ion permeable barrier in electrochemical cells. It has high mechanical strength, improved mechanical and chemical stability, and higher ion permeability. Our separtor material has proven to be exceptionally resistant to penetration by zinc dendrites, making this material particularly useful in secondary batteries having zinc anodes. Our separtor material is readily and thoroughly wettable by alkaline electrolytes, yet has an extremely high resistance to chemical attack by them. In addition, wet strength of our new separator material is exceptionally high, making further improvements in a zinc-air hybrid fuel cell possible.